Piezoelectric crystals have been used for many decades as frequency control elements in radio communication devices because of their stable resonant frequency signal generation during operation. Generally, the resonant frequency of a particular piezoelectric crystal is tuned to a desired frequency by changing the mass loading of the crystal. Typically, this has been accomplished by adding or removing mass from the electrode portion of the crystal. As the resonant frequency is very sensitive to changes in mass loading, small changes in mass can change frequency dramatically. Mass can also be added or removed from the piezoelectric material directly. However, it is much more difficult to add or remove the piezoelectric material, typically a dielectric, than it is to add or remove electrode material, typically a metal.
Prior art methods of adding mass to a crystal have included vacuum evaporative deposition of a metal to the electrode of the crystal. Typically, this can be done at a very high rate of speed. In addition, adding material to an electrode of a crystal does not alter, change or damage the electrode to any significant degree. As a result, there is very little detriment to the performance of the crystal. However, electrical shorting is possible between multiple resonators.
Prior art methods of removing mass from a crystal, such as laser or ion milling for example, have proven more difficult to do successfully, in particular where high throughput is desired. Removing material from the electrode or the piezoelectric material causes damage, which detrimentally affects the performance of the crystal. This gives rise to higher resistance devices, intermodulation distortion, and poor aging characteristics. In addition, some metals such as aluminum form an oxide layer, even under typical production vacuum levels. Oxide layers are much harder to remove at an adequate rate unless high power is used. However, high power accelerates any damage being done to the crystal and heats the crystal making it difficult to accurately measure frequency while processing the crystal. Moreover, mass removal is non-linear, in that, once the oxide layer is penetrated, mass removal rates changes significantly. Therefore, mass removal techniques have proven hard to control.
To be sure, there are metals that do not oxide significantly, such as the noble metals of gold and silver. However, these metals are typically very heavy and can not be used for higher frequency resonators due to extreme mass loading problems. In addition, metals such as gold and silver require more metal thickness to become usefully conductive than do lighter weight metals such as aluminum. As is recognized in the art, high frequency devices, in particular fundament devices, require the use of lightweight electrodes to prevent adverse spurious frequency modes from occurring.
There is a need for a piezoelectric resonator that can be tuned by mass removal at a relatively high rate, with little degradation of device performance. There is also a need for a piezoelectric resonator that can be tuned by mass removal at a substantially linear rate, and that can be realized in a simple, readily manufacturable form, at a low cost and high yield.